The relationships between longevity and cumulative energy metabolism in homeothermic vertebrates are explored interspecifically as allometric functions, and intraspecifically in terms of metabolic extension, through restriction of energy intake, hibernation, and torpor. A new equation for marsupial longevity is presented. The first evidence of age-dependent mortality in a population of hummingbirds is shown. Hummingbirds are seen to be model subjects for the study of longevity in nature.